1. Field of the Invention
The present invention relates to an apparatus and method for local pressurizing type injection molding.
2. Description of the Related Art
In a conventional injection molding machine, a screw is disposed in a heating cylinder for rotational and reciprocating movement with respect to the heating cylinder. In a metering stage, the screw is retracted while being rotated so as to accumulate a molten resin in front of the screw, and in a charging stage, the screw is advanced to inject the molten resin into a cavity in a die through an injection nozzle, thereby filling the cavity with the molten resin.
The die is composed of a stationary die and a movable die. The movable die is advanced and retracted by a mold opening/closing apparatus so that the movable die contacts and separates from the stationary die. Resin is charged into the cavity after mold closing, in which the movable die is contacted with the stationary die, and mold clamping, in which the movable die is pressed against the stationary die. After cooling, the resin becomes a molded product. Subsequently, mold opening is effected for separating the movable die from the stationary die, and the molded product is thrust out by projecting an ejector pin from the stationary die.
In the resin charging stage, the resin injected from the injection nozzle enters a runner through a sprue and is then charged into the cavity through a gate. In the case where the gate is a submarine type, a portion of the molded product corresponding to the gate (hereinafter referred to as a "gate portion") is automatically broken and is separated from the molded product when the molded product is thrust out. However, a mark formed due to the gate (hereinafter referred to as "gate mark") or flashes are left on the molded product from which the gate portion has been separated. Accordingly, further processing is needed to remove the gate mark or flashes, resulting in increased costs.
To obviate the above-described problem, a local pressurizing type injection molding machine has been proposed in which a pressurizing pin is disposed at a location corresponding to the gate portion and is advanced to close the gate. In this case, generation of a gate mark and flashes can be prevented.
In addition, it is possible to dispose another pressurizing pin other than the pressurizing pin for gate-closing to perform a processing operation on a molded product as well as to perform gate-closing. However, the timing at which the gate-closing is started cannot be made different from the timing at which the processing operation is started. Similarly, the pressurizing force for the gate-closing cannot be made different from the pressurizing force for the processing operation.
If a pressurizing pin for gate-closing and another pressurizing pin attached to the same pressurizing plate are simultaneously advanced, in addition to gate-closing, other actions can be performed such as, for example, prevention of the formation of a sink due to contraction of the resin upon cooling, punching of a hole, formation of a thin wall portion, and compression molding for an optical part.
To make the surface corresponding to the gate smooth, it is necessary to start the gate-closing at a relatively early timing. On the contrary, to prevent the formation of a sink, it is preferred to impose pressure on the resin as the cooling process proceeds, and is necessary to start the gate-closing at a relatively late point in time.
Since the pressurizing pin for gate-closing and the other pressurizing pin are mounted on a single pressurizing plate and are advanced by advancing movement of the pressurizing plate, the timing for the gate-closing becomes the same as the timing for another processing operation.
Moreover, the pressurizing force for gate-closing is the same as that for the processing operation, because the pressurizing plate is advanced by a single pressurizing cylinder.